The embroidery frames used for locally clamping fabrics on embroidery heads of embroidery machines consist of a closed inner hoop, and a slotted outer hoop which is connected at its ends by a tensioning device and surrounds the fabric of the inner hoop with an intermediate gap. The hoops are made of the same material, such as wood or plastic. To process embroidery patterns of different geometric size, embroidery hoops with round, elliptical and even nearly rectangle-like outer contours with rounded corners are used.
The tensioning device, arranged on the outer hoop outside the symmetry plane, serves to set the holding force between the inner hoop and the outer hoop and permits adaptation to different fabric thicknesses.
Difficulties due to slippage of the fabric as a consequence of the nonuniform distribution of the contact pressures between the inner hoop and the outer hoop, which depends on the local radius of curvature, often occur in the case of the non-circular embroidery hoops that are very commonly used. For example, contact pressure is concentrated in the areas of small radii of curvature, whereas in other areas the contact pressure may drop to such an extent that sufficient clamping of the fabric to be embroidered is not guaranteed.
To avoid these difficulties, circular embroidery hoops with rectangular cross section of the hoop parts in defined diameter steps are known, in order to thus eliminate the effect of different radii of curvature. However, the use of circular tensioning hoops is not advantageous for certain, e.g., elongated embroidery patterns.